1. Field of the Invention
The invention relates to a friction apply device of a vehicular automatic transmission. More particularly, the invention relates to the structure of a clutch piston which constitutes a friction apply device.
2. Description of the Related Art
One vehicular automatic transmission includes a bottomed cylindrical clutch piston as a clutch piston that applies pressure to a friction apply element that includes a plurality of friction plates when a clutch and brake provided in the automatic transmission are applied. This clutch piston is known to reduce costs compared with an integrally formed clutch piston because it is formed of a plurality of members. For example, a clutch piston structure having a cylindrical bottom described in Japanese Patent Application Publication No. JP-A-9-32919 includes three members: a cylindrical cylinder member 104, a disc-shaped bottom member 105, and a snap ring 106, as shown in FIG. 6. A plurality of notches 107 are formed at intervals in the circumferential direction in one end of the cylinder member 104. Pawl-shaped portions 108 are formed in the outer peripheral edge of the bottom member 105 in positions which enables them to fit into the notches 107 in the cylinder member 104. The structure is then integrated by first fitting the pawl-shaped portions 108 of the bottom member 105 into the notches 107 in the cylinder member 104, and then fitting the snap ring 106 into an inner peripheral groove 109 formed in the inner peripheral surface of the cylinder member 104.
The inner peripheral groove 109 of the cylinder member cylinder member 104 into which the snap ring 106 fits is formed in the end portion of the bottomed cylindrical clutch piston of the foregoing structure, and results in the cylinder member 104 being thinner at that portion. When the clutch is applied, force in a direction that pulls parallel to the axial center of the cylinder member 104 is applied to this thin end portion via the snap ring 106 in the inner peripheral groove 109 so there is a need to make the end portion of the cylinder member 104 in which the notches 107 are formed comparatively stronger.
On the other hand, the bottomed cylindrical clutch piston is relatively heavy so attempts are being made to try to reduce its weight by forming it out of an aluminum alloy. When the piston is made of an aluminum alloy, however, the durability of the end portion of the cylinder member 104 in which the notches 107 are formed decreases because aluminum alloy is not as strong as steel.